Corrosion resistant aluminum alloys

ABSTRACT

A corrosion resistant aluminum alloy based on commercial purity aluminum with deliberate additions of silicon and manganese and with a restricted iron content. The silicon and manganese ranges are controlled to provide an aluminum-silicon solid solution and to restrict or eliminate cathodic particles which have been found to cause pits. The alloy is highly resistant to corrosion, especially in environments where the alloy comes into contact with impure water.

[ Aug. 27, 1975 CORROSION RESISTANT ALUMINUM 3,639,107 2/1972 Thompson 75/138 ALLOYS [75] Inventors: William H. Anthony; Popplewell, Primary Dean both of Gullford' Conn Attorney, Agent, or FirmRolbert H. Bachman; David 731 Assignees Swiss Aluminum Limited, Chippis, Jackson Switzerland [22] Filed: Mar. 14, 1974 211 Appl. No.: 451,074 [57] ABSTRACT .Related Application Data A corrosion resistant aluminum alloy based on com- Continuation-impart 0f 12, mercial purity aluminum with. deliberate additions of 1973 3,87887L silicon and manganese and with a restricted iron conv tent. The silicon and manganese ranges are controlled [52] US. Cl. 148/32; 75/141; 74/146; to provide an alnminnm si]icon Solid Solution and to 75/148 restrict or eliminate cathodic particles which have [51] Int. Cl. C22C 21/02 been f d to Cause nits. The alloy is highly resistant [58] Field of Search 75/141, 142, 143, 138, to Corrosion, especially in environments where the 75/1471 146; 148/327 alloy comes into contact with impure water.

[56] References Clted 7 Claims, 1 Drawing Figure UNITED STATES PATENTS 3,219,492 11/1965 Anderson et al. 75/138 LEGEND PITT/N6 16- WE/GHILOSS I6 N v) HIGH PUP/TY 6 g -10 g 1 k S 9. a I w Q a g Q =1 w 6 o s; 3 C 4 aoa%s1 H267 Mn, 2 2

HIGH FUR/TV A1 a O 008681406414. e-,ez Ti: a 50 /20 150 EXPOSURE TIME IN Uni s AVE/PAGE P/r DEPTH M/LS EXPOSURE 77MB IN DAYS US. Patent Dec. 2, 1975 3,923,557

/a-- LEGEND PITT/N6 l6 WIS/6H1 LOSS /6 I 3003 12.- V g, l2

/ HIGH PUR/TV 2 HIGH PUR/TV A! n 2 Two g "j WEIGHT Loss M/LL/GRAMS PER C7112 CORROSION RESISTANT ALUMINUM ALLOYS CROSS REFERENCE TO RELATED APPLICATION This case is a continuation-in-part of copending ap-' plication Ser. No. 414,862, by William H. Anthony and James M. Popplewell for A Corrosion Resistant Aluminum Composite, filed Nov. 12, 1973 now US. Pat. No. 3,878,871.

BACKGROUND OF THE INVENTION Many industrial processes result in the formation of a large amount of waste steam. Economic considerations require that the heat content of this steam should be recovered by condensing the steam. This condensation process is performed by passing the steam over metal tubes through which cooling water is passed. The cooling water is commonly impure, and contains impurities which cause severe corrosion problems and thereby add to the expense of the industrial process. Such condensors contain large quantities of tubing and represent a large potential market for any alloy which can withstandthe corrosion effects of the cooling water. The preferred materials at present are stainless steel, and admiralty brass. Unfortunately, materials used heretofore in condensor applications have not been entirely satisfactory when considered from a price-performance viewpoint. Aluminum has not received much consideration because previously considered alloys have not had an adequate combination of resistance to pitting and general corrosion.

Many other industrial processes are also candidates for the application of a more corrosion resistant tubing of moderate cost. Examples include oil refinery and general chemical industry piping and tanks, irrigation equipment for agriculture, and automotive radiator applications.

SUMMARY OF THE INVENTION DESCRIPTION OF THE PREFERRED EMBODIMENTS The composition of the alloy of the present invention in the following description of the preferred embodiments is given in weight percentages unless otherwise specified.

The broad and preferred composition limits for the alloy of the present invention are given in Table I below:

TABLE I Broad Preferred Silicon 0.05 0.5 0.15 0.25 Manganese 0.2 0.8 0.3 0.6 Iron 0.0 0.2 0.0 0.08 Chromium 0.0 0.1 0.0 0.05 Magnesium 0.0 0.3 0.0 0.1 Copper 0.0 0.1 0.0 0.05 Titanium 0.0 0.05 0.005 0.015

TABLE I-continued Broad Preferred Zinc 0.0 0.1 0.0 0.05

The essential components of the alloy are manganese and silicon. The remaining elements may be present as impurities up to the level shown in Table I. Titanium may be present as a purposeful addition for grain refinement of the alloy. The zinc has not been found to have any detrimental effects on the corrosion behavior of the alloy and its level has been chosen to permit the use of zinc bearing scrap in the production of the alloy. Naturally, any of the foregoing impurities may be present in levels as low as 0.001%.

A major cause of pitting corrosion in aluminum alloys is the presence of particles in the alloys which are cathodic or anodic to the matrix of the alloy. Such particles act to set up galvanic cells when the alloy is in a conducting medium. Such cells act as initiation sites for the formation of pits. Particles which are likely to cause pitting include; silicon, FeAl CuAl MnAl and a(Al- FeSi). In the course of the present research it was found that the presence of a cathodic particle of FeAl in a 6061 Alloy could lead to "the formation of a pit in as little as one hour when the alloy was exposed to flowing tap water at a" temperature of 30C.

The alloy composition of the present invention has been selected on the basis of the constitutional relationship which has been derived for the aluminummanganese-silicon-iron quaternary system by H. W. L. Phillips, Journal of the Institute of Metals, Vol. LXIX, 1943. From this phase diagram it appears that the presence of about 0.4% manganese will surpress the formation of the FeAl particles. However, particles of a(Al- FeSi) remain. For this reason the iron concentration in the present invention has been limited, since a(AlFeSi) particles are also detrimental to the corrosion behavior of aluminum alloys, although to a much lesser degree than FeAl; particles.

Other research has indicated that the pitting resistance of aluminum can be greatly improved by the introduction of silicon, provided that the silicon is in solid solution. The concentration range over which the silicon will remain in solid solution is largely dependent upon the iron and manganese concentrations, and the presence of magnesium. The silicon levels of the alloy of the present invention have been chosen in light of these considerations.

The preceeding discussion of the present invention will be better understood through consideration of the following illustrative examples:

EXAMPLE I A series of castings were made using high purity aluminum as a base. The high purity aluminum contained the following impurities; 0.001% iron and 0.001% silicon. To this base the following deliberate additions were made:

A. 0.08% silicon B. 0.6% manganese 0.08% silicon c. No addition The castings were homogenized at 1l00F for 16 hours and were air cooled. The ingots were then scalped and hot rolled from 1.5 inches to 0.25 inch at a starting hot rolling temperature of 825F. The ingots for weight loss and pit depth. The data is displayed in FIG. 1. These results demonstrate the definite superiority of the alloy of the present invention over high purity aluminum and the commercial alloy, 3003 control sample. It is evident that a combination of manganese and silicon results in an alloy superior in both overall corrosion rate and pit depth. Table 11 shows the approximate analysis of the water used in the examples.

TABLE II 4 inch. The alloys were tested according to the procedure described in Example I except that the water was replenished every 12 hours. An approximate analysis of the water used is given in Table 11.

Examination of the corrosion data which is presented in Table 111 indicates that there is a type of synergistic effect on the pitting resistance over certain ranges of silicon and manganese. The optimum manganese level apparently lies between 0.4 and 1.0% and preferably near 0.4% while the optimum silicon level is at least 0.2%. The presence of 0.2% chromium has an adverse effect on the pitting resistance as does the presence of 1.0% magnesium, while manganese has a beneficial effect up to 0.6% but detrimental at the 1% level. Therefore 0.8% seems to be the cut off point in usefulness. The detrimental effect of magnesium is due to the depletion of the solid solution concentration of silicon by WATER ANALYSIS (PPP) Al CONDENSER TUBING PROJECT New Haven Test with Continuous Test with lnterrnittent Tap Water Refreshment with New Haven Tap Water Refreshment Once a Week Fe 1 Cu .02 .05 0.01 pH 7.2 6.8 7.8 0, 7.5 12.0 8.4 CO1 5.0 3.0 2.0 Solids 90.0 109.0 120.0 Hardness (CaCO 55.8 37.8 68.2 Alkalinity (CaCO 40.5 19.3 58.5 Calcium 35.7 14.2 32.2 Sodium 5.0 4.0 7.0 Sulfate 60.0 65.0 47.0

Example I employed a test in which the water was changed only once a week and it is evident that over a period of one week the chloride concentration decreases significantly. Because of the corrosive effect of chloride ions, subsequent tests were performed using continuous replenishment of the tap water in order to increase the severity of the test.

EXAMPLE II A series of experimental ingots were cast using a high purity aluminum base but containing from 0.05 to 0.063% iron. The composition of these alloys is given in Table III along with 60 day corrosion data.

TABLE III EXAMPLE III Three alloys having a base composition of 0.1% silicon and 0.6% manganese were cast and processed to CORROSION TEST RESULTS AND SPECTROSCOPIC ANALYSES ON EXAMPLE I1 ALLOYS AFTER DAYS EXPOSURE TO NEW HAVEN TAP WATER Percentage Composition Pit Depth (mils) Weight Loss Fe Si Mn Cr Mg Mean Max mlgms/cm .056 .1 l .43 15.0 22.5 12.8 .051 .23 .31 8.4 17.8 15.1 .055 .23 .45 8.8 15.3 15.4 .053 .10 .64 10.2 19.7 14.2 .050 .03 1.0 13.3 20.2 g 17.6 .061 .10 .63 .20 13.2 18.0 13.1 .061 .098 .61 21 1.04 16.7 22.4 10.3 3003 Control 27.1 17.7

27.1 mill thick coupon was perforated The castings were homogenized at 1100F for 16 hours and water quenched. They were subsequently scalped and hot rolled from 1.5 inches to 0.175 inch using a starting hot rolling temperature of 825F. The alloys were then cold rolled to 0.072 inch and given a 2 hour anneal at 650F with a controlled cooling rate of 50F/hr down to 400F, and then cold rolled to 0.05

II. The results are shown in Table IV and indicate that countered, whether in power plants or petroleum refinthe alloy consisting of deliberate additions of 0.1% 5111- eries.

TABLE V 2% Si Mn Fe Cr Mg Cu Y.S. U.T.S. Elong.

con and 0.6% manganese perform approximately as well from a pitting standpoint after exposure of 60 and 180 days, regardless of whether the iron content is In view of the well known highly detrimental effect of iron on pitting resistance of aluminum alloys, the efficacy of the manganese addition is clearly demonstrated. The addition of the titanium diboride in an amount sufficient to be effective as a grain refiner had no detectable effect on the pitting resistance of the alloy.

TABLE IV CORROSION TEST RESULTS AND SPECTROSCOPIC ANALYSES OF EXAMPLE lll ALLOYS EXPOSED FOR VARIOUS TIMES UP TO l80 DAYS IN NEW HAVEN TAP WATER Weight Loss Pit Depth (mils) mlgms/cm' Percentage Composition 60 Days 120 Days 180 Days 60 120 180 Fe Si Mn Ti B Mean Max Mean Max Mean Max Days IDays Days consisting essentially of from. 0.05 to 0.5% silicon, wherein the silicon is resent in solid solution from 0.2 EXAMPLE IV p The alloys of the present invention have moderate mechanical properties. Alloys having compositions as listed in Table V were cast and processed according to the process described in Example 11. The final cold reduction was 30% and the resultant mechanical properties are listed in Table V.

The alloys of the present invention have a wide potential area of usefulness, encompassing almost any application in which a metallic article must come into contact with relatively impure water or other aqueous media. Typical of such applications are tubing or piping for the flow of aqueous media and heat exchangers for the transfer of heat to or from an aqueous medium. The alloy of the present invention is particularly suitable for the fabrication of thin wall tubing, as for example welded tubing formed from metal strips. Such tubing would normally have a wall thickness of from 0.02 to 0.375 inch depending upon the tube diameter, and a diameter of from 16 inch to 16 inches. Of course, thick wall tubing and piping may be fabricated having a wall thickness of as much as 1.0 inch. The alloy of the present invention may also be used in the fabrication of items such as tube sheets, and tube spacers and supports. In general, the alloy of the present invention is useful whenever aqueous corrosion problems are ento 0.8% manganese, from 0.001 to 0.2% iron, from 0.001 to 0.1% chromium, from 0.001 to 0.1% magnesium, from 0.001 to 0.1% copper, from 0.001 to 0.05% titanium, from 0.001 to 0.1% zinc, balance aluminum.

2. An alloy according to claim 1 wherein the silicon is from 0.15 to 0.25%.

3. An alloy according to claim 1 containing less than 0.4% manganese.

4. An alloy according to claim 1 wherein the iron content is from 0.001 to 0.08%.

5. An alloy according to claim 1 containing from 0.001 to 0.05% chromium, from 0.001 to 0.05% copper, from 0.005 to 0.015% titanium, and from 0.001 to 0.05% zinc.

6. An alloy according to claim 1 wherein the magnesium content is less than 0.01%.

7. An aluminum base alloy article highly resistant to pitting and corrosion in an aqueous environment wherein the alloy consists essentially of from 0.05 to 0.5% silicon, wherein the silicon is present in solid solution, from 0.2 to 0.8% manganese, from 0.001 to 0.2% iron, from 0.001 to 0.1% chromium, from 0.001 to 0.1% magnesium, from 0.001 to 0.1% copper, from 0.001 to 0.05% titanium, from 0.001 to 0.1% zinc, balance aluminum.

Nut-,1) STALLQ PALM?! Alum TLhADlLl'w/ruw. FICEJ QERTIFICATE 0F CQRREQTION PATENT NO. 3,9 3,557

DATED December 2, 1975 INVENTOR(S) William H. Anthony and James M. Popplewell It is certified that error appears in the above-identified patent and that said Letters Patent hereby corrected as shown below In the heading, before "Popplewell" insert ---James M.---;

P In the heading, "Swiss Aluminum Limited" should read ---Swiss Aluminium Limited.

Page 1, Issue Date of Patent "Aug. 27, 1975" should read Dec. 2, l975.

lgncd and Scaled thusthirtieth D f March 1976 [SEAL] Attest:

RUTH C. MASON c. MARSHALL DANN Anem'lg 011w Commissioner uflarenrs and Trademarks 

1. A CORROSION RESISTANT ALUMIN BASE ALLOY RESISTANT TO PITTING AND CORROSION IN AN AQUEOUS ENVIRONMENT CONSISTING ESSENTIALLY OF FROM 0.05 TO 0.5% SILICON, WHEREIN THE SILICON IS PRESENT IN SOLID SOLUTION, FROM 0.2 TO 0.8% MANGANESE, FROM 0.001 TO 0.2% IRON, FROM 0.001 TO 0.1% CHROMIUM, FROM 0.001 TO 0.1% MAGNESIUM, FROM 0.001 TO 0.1% COPPER, FROM 0.001
 2. An alloy according to claim 1 wherein the silicon is from 0.15 to 0.25%.
 3. An alloy according to claim 1 containing less than 0.4% manganese.
 4. An alloy according to claim 1 wherein the iron content is from 0.001 to 0.08%.
 5. An alloy according to claim 1 containing from 0.001 to 0.05% chromium, from 0.001 to 0.05% copper, from 0.005 to 0.015% titanium, and from 0.001 to 0.05% zinc.
 6. An alloy according to claim 1 wherein the magnesium content is less than 0.01%.
 7. An aluminum base alloy article highly resistant to pitting and corrosion in an aqueous environment wherein the alloy consists essentially of from 0.05 to 0.5% silicon, wherein the silicon is present in solid solution, from 0.2 to 0.8% manganese, from 0.001 to 0.2% iron, from 0.001 to 0.1% chromium, from 0.001 to 0.1% magnesium, from 0.001 to 0.1% copper, from 0.001 to 0.05% titanium, from 0.001 to 0.1% zinc, balance aluminum. 